[GRASSLIST:6375] Re: r.terraflow and r.water.outlet
swlab at cornell.edu
Fri Apr 8 12:20:47 EDT 2005
> No. I tried some time ago with no success. The flow directions do not
> seem to be what r.water.outlet expects.
I gonna simplify a bit, that should give you the gist of it nevertheless
r.terraflow uses by default a MFD algorithm (multiple flow path). In short,
the outflow of one cell is distributed among ALL its downslope neighbors,
according to the relative differences in elevation. The nb stored in the flow
direction output of terraflow corresponds to the neighbors id, stored as an
octet (so, between 0 and 255), clockwise from East (1 for East, 2 for SE, 4
for S, 8 for SW...) : for example, if water flows to the North and the south
simultaneously, the flow direction value is 68 (4 for S + 64 for N). NNE ?
64+128, and so on...
r.watershed uses a SFD (single flow direction) algorithm, where water is
routed to ONE downslope neighbor. The flow directions are coded as an integer
between 1 and 8.
There should be a trick to get main flow directions (pseudoSF) from terraflow
MFD: check the sources, direction.cc: there's a series of test to get the
main direction from the 0-255 value (function findDominant). A simple script
should do the work. I tried to write such a script smoe time ago, but don't
ask me where it is, my box is a mess these days...
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